Batteries Phenomena and Mechanism
Batteries may be divided into two principal types, primary batteries and secondary batteries. Primary battery can be used once until it discharges. Secondary batteries are also called rechargeable batteries because they can be recharged to use again. In secondary batteries, each charge/discharge process is called a cycle, and eventually reaches an end of their usable life after many charge/discharge cycles. Electrochemical cell constitutes of a cathode (positive) and anode (negative) electrodes, an electrolyte (an ionic conductor) to chemically connect the electrodes, and an insulator separating the electrodes. In actual operation, the secondary battery converts the chemical energy into electrical energy. During discharging of battery, negative charge (electrons) leave the anode and move in external circuit, while the positive charge (Li-ion) travel through the electrolyte to reach cathode. During charging, Li-ion moves from the cathode to anode via electrolyte depending on the electrochemical potential (ECP) between the cathode and anode to maintain charge neutrality. Charge-discharge may be a complex process due to the nature of cathode, anode, and electrolytes. Li—S cathode is very poor conductor due to the insulating nature of sulfur which causes capacity fading. Therefore, high conductivity is an essential requirement. Since carbon is highly conductive, S—C composite is an obvious choice, but creation of conduction link between C and S is crucial. Nano-links between carbon and sulfur can provide the conducting channels, if one could synthesis them. Nano-links facilitate conduction of the Li-ion in Li—S cathode. The discloser provides the synthesis routes of C—S nanocomposite with nano-network which is highly conductive with long cycle life in actual testing with assembled battery.
Unlike most rechargeable batteries, where ion moves via intercalations into and out of a crystal lattice, lithium-sulfur batteries are based on topotactic chemical reaction at the anode and cathode (16 Li+S8⇔Li2S). But, theoretical capacity (1675 mAh/g) of Li—S cathode is much higher than that of the oxide based cathodes (<200 mAh/g). Li—S also operates at a safer voltage range (1.5 V to 3 V), but the poor conductivity of Li—S is very serious problem. High theoretical capacity of Li—S is due to the ability of sulfur to accept two electrons per atom. The high-order polysulfides (Li2Sn, 4<Sn<8) produced during the initial stage of discharge process, are soluble in the electrolyte and move toward the lithium metal anode, where they are reduced to lower-order polysulfides. Moreover, solubility of these high-order polysulfides in the liquid electrolytes and the nucleation of insoluble low-order sulfides (e.g. Li2S2 and Li2S) results in poor capacity retention and low coulombic efficiency. Since carbon is an excellent electric conductor, C—S material system was an obvious choice do develop conductive cathode. Based on this fact various investigators have intensively studied the routes for the synthesis of C—S material system.
Several synthesis approaches have been pursued including composites with carbon black and nanostructured carbon with considerable capacity fading. But, Li—S synthesized by mesoporous carbon with amorphous sulfur and polymer have exhibited high reversible capacity of approximately 1000 mAh/g up to 20 cycles. Most traditional methods to synthesize sulfur-carbon composites include processing by sulfur melting route, resulting in high manufacturing costs due to additional energy consumption. Several reports have noted that the sulfur content in the sulfur-carbon composites synthesized by the sulfur melting route is limited to a relatively low specific capacity to obtain acceptable electrochemical performance, which leads to a lower overall capacity of the cathode. Synthesizing homogeneous sulfur-carbon composites through conventional heat treatment is complicated and still challenging. In this method of synthesis, sulfur is heated above its melting temperature, and the liquid sulfur is then diffused to the surface or into the pores of carbon substrates to form the sulfur-carbon composite. A subsequent high-temperature heating step is then required to remove the superfluous sulfur on the surface of the composites, and leads to waste some sulfur. Thus, the conventional synthesis by the sulfur-melting route may be difficult for scaling to obtain a uniform industry level demand for sulfur-carbon composite. Recently, another alternative of sulfur deposition to synthesize core-shell carbon/sulfur material for Li—S batteries was introduced. The capacity fading after one cycle (solubility of polysulphides) was the clear result. The carbon-sulfur composite material where sulfur particles are wrapped by polyethylene glycol (PEG) and carbon sheets showed stable capacities of 600 mAh/g over 100 cycles. Also, one-port reaction has been developed to obtain carbon-enveloped sulfur composite which had high sulfur content of 87 wt % with acceptable cycling stability. Despite the fact that carbon-sulfur network was found effective to enhance the cycle performance of Li—S batteries, its synthesis is usually complicated and challenging for large-scale application. Most of the solution-based synthesis without heat treatment produces large crystalline sulfur particles with poor rate performance due to lack of the full utilization of active material. Thus, there remains urgent need for a highly functional and readily synthesized sulfur-carbon cathode material for Li—S battery. Nano-network links are very effective to provide new electrical paths to Li-ion. We used this concept for the material synthesis and the performance of Li—S cell assembled with this nano-composite cathode with battery capacity >910 mAh/g up to 80 cycles.